Radiographic equipment

ABSTRACT

The invention concerns radiographic equipment. The equipment includes a source of substantially mono-energetic fast neutrons produced via the deuterium-tritium or deuterium-deuterium fusion reactions, comprising a sealed-tube or similar generator for producing the neutrons. The equipment further includes a source of X-rays or gamma-rays of sufficient energy to substantially penetrate an object to be imaged and a collimating block surrounding the neutron and gamma-ray sources, apart from the provision of one or more slots emitting substantially fan-shaped radiation beams. Further included is a detector array comprising a multiplicity of individual scintillator pixels to receive radiation energy from the sources and convert the received energy into light pulses, the detector array aligned with the fan-shaped beams emitted from the source collimator and collimated to substantially prevent radiation other than that directly transmitted from the sources reaching the array. Conversion means are included for converting the light pulses produced in the scintillators into electrical signals. Conveying means are included for conveying an object between the sources and the detector array. Computing means are included for determining from the electrical signals the attenuation of the neutrons and the X-ray or gamma-ray beams and to generate output representing the mass distribution and composition of the object interposed between the source and detector array. The equipment further includes a display means for displaying images based on the mass distribution and the composition of the object being scanned.

This application is a 371 of PCT/AU2003/001641, filed Dec. 10, 2003; thedisclosure of which is incorporated herein by reference.

TECHNICAL FIELD

This invention concerns radiographic equipment. In particular theinvention concerns radiographic equipment for the detection of concealedarticles, substances and materials. For instance, the invention may beapplied to the detection of concealed weapons, explosives, contraband,drugs and other articles, substances and materials in items such asaircraft baggage, airfreight or shipping containers.

BACKGROUND ART

Technologies based on X-rays, gamma-rays and neutrons have been proposedto tackle this problem (Hussein, E., 1992, Gozani, T., 1997, An, J. etal, 2003). The most widely adopted technology is the X-ray scanner whichforms an image of an item being examined by measuring the transmissionof X-rays through the item from a source to a spatially segmenteddetector. X-rays are most strongly attenuated by dense, higher atomicnumber materials such as metals. Consequently, X-ray scanners are idealfor detecting items such as guns, knives and other weapons. However,X-rays provide little discriminating power between organic and inorganicelements. Using X-rays the separation of illicit organic materials suchas explosives or narcotics from commonly found, benign organic materialsis not possible.

An elemental identification system is being developed for the inspectionof commodities shipped on pallets. The system called NELIS (NeutronElemental Analysis System) utilises a 14 MeV neutron generator and threegamma ray detectors to measure induced gamma rays from the cargo(Dokhale, P. A. et al, 2001; Barzilov, A. P., Womble, P. C. andVourvopoulos, G., 2001). NELIS is not an imaging system and is used inconjunction with an X-ray scanner to help identify gross compositionanomalies.

A Pulsed Fast Neutron Analysis (PFNA) cargo inspection system has beendeveloped (Gozani, T., 1997, Sawa et al., 1991) and commercialisedthrough Ancore Corporation. The PFNA system uses a collimated beam ofnanosecond-pulsed fast neutrons and the resulting spectrum of gamma raysis measured. The PFNA method allows the ratios of key organic elementsto be measured. The nanosecond-pulsed fast neutrons are required inorder to localise the specific regions contributing to the measuredgamma-ray signal by time-of-flight spectrometry. In practice thetechnique is limited by the very expensive and complex particleaccelerator, the limited neutron source strength and low gamma-raydetection efficiency and the resulting slow scan speeds.

Neutron radiography systems have the advantage of direct measurement oftransmitted neutrons and are therefore more efficient than techniquesmeasuring secondary radiation such as neutron-induced gamma rays. Fastneutron radiography has the potential to determine the line-of-sight‘organic image’ of objects (Klann, 1996). In contrast to X-rays,neutrons are most strongly attenuated by organic materials, especiallythose with high hydrogen contents.

A fast neutron and gamma ray and radiography system has been developedby Rynes et al (1999) to supplement PFNA. In this systemnanosecond-pulsed fast neutrons and gamma rays from an accelerator aretransmitted through the object and the detected neutron and gamma raysignals are separated by arrival time. The resulting system is claimedto combine the advantages of both X-ray radiography and PFNA systems.However it is limited by the very expensive and complex particleaccelerator.

Bartle (1995) has suggested using the fast neutron and gamma-raytransmission technique (Millen et al, 1990) to detect the presencecontraband in luggage, etc. However this technique has not been used forimaging and its practical application to contraband detection has notbeen investigated.

Mikerov, V. I. et al, (2000) have investigated the possibility of fastneutron radiography using a 14 MeV neutron generator and luminescentscreen/CCD camera detection system. Mikerov found that applications werelimited by both the low detection efficiency of the 2 mm thickluminescent screen for fast neutrons and the high sensitivity of thescreen to X rays produced by the neutron generator.

Neutron radiography systems using a 14 MeV generator and thermal neutrondetection are commercially available (Le Tourneur, P., Bach, P. andDance, W. E., 1998). However the fact that the fast neutrons are sloweddown (thermalised) prior to performing radiography limits the size ofthe object being imaged to a few cm. No fast neutron radiography systemsare commercially available that involve fast neutron detection.

Most work conducted with neutron radiography has been conducted in thelaboratory using neutrons from nuclear reactors or particle acceleratorsthat are not suited to a freight-handling applications (Lefevre, H. W,et al, 1996, Miller, T. G., 1997, Chen, G. and Lanza, R. C., 2000,Brzosko, J. S. et al, 1992).

To improve the ability of fast neutron radiography systems to providediscrimination between various organic materials, systems using multipleneutron energy sources, together with detectors with the means fordistinguishing between the different neutron energies have been proposed(Chen, G. and Lanza, R. C., 2000, Buffler, 2001). The key drawbacks ofthese systems have been their reliance on complex, energy-discriminatingneutron detectors and/or their use of sophisticated, high-energyaccelerator-based neutron sources.

Perion et al. (Perion, 2000) have proposed a scanner using a high-energy(MeV) X-ray Bremsstrahlung or radioisotope source. By either modulatingthe average source energy by rapidly inserting and removing a low atomicnumber filter, or by measuring the energy of detected X-rays, it ispossible to measure transmission through the object being scanned overtwo different X-ray energies, one where Compton scattering dominates andone where pair-production is significant. This information can be usedto deduce the density and average atomic number of material in eachpixel of the scan image. The main drawback of this scheme is the lowcontrast between different elements, even when very high energy X-raysources are used. The cost of the Perion detector array would also bevery high. Alternatively, Perion suggests that measurement of thetransmission of both X-rays and neutrons (produced either directly inthe Bremsstrahlung target or by inserting a neutron-producing filter)can yield similar information. The main disadvantage of this method isthe low energy of neutrons produced via (gamma, n) reactions. Thislimits the ability of the neutrons to penetrate through thick cargoesand increases the difficulty in adequately detecting the transmittedneutrons. In particular, it is unlikely that the disclosed stackedscintillator detector would be able to distinguish neutrons in thepresence of a much more intense X-ray beam. A disadvantage of both thedual energy X-ray and the X-ray/neutron schemes is that the X-rays andneutrons cover a wide range of energies. This means that it is notpossible to model transmission using a simple exponential relation andthat it is not straightforward to extract quantitative cross-sectioninformation that could be used for material identification.

DISCLOSURE OF INVENTION

The present invention is radiographic equipment comprising:

a source of substantially mono-energetic fast neutrons produced via thedeuterium-tritium or deuterium-deuterium fusion reactions, comprising asealed-tube or similar generator for producing the neutrons;

a source of X-rays or gamma-rays of sufficient energy to substantiallypenetrate an object to be imaged;

a collimating block surrounding the neutron and X-ray and gamma-raysources, apart from the provision of one or more slots for emittingsubstantially fan-shaped radiation beams;

a detector array comprising a multiplicity of individual scintillatorpixels to receive radiation energy emitted from the sources and convertthe received energy into light pulses, the detector array aligned withthe fan-shaped radiation beams emitted from the source collimator andcollimated to substantially prevent radiation other than that directlytransmitted from the sources reaching the array;

conversion means for converting the light pulses produced in thescintillators into electrical signals;

conveying means for conveying the object between the sources and thedetector array;

computing means for determining from the electrical signals theattenuation of the neutrons and the X-ray or gamma-ray beams and togenerate output representing the mass distribution and composition ofthe object interposed between the sources and detector array; and

display means for displaying images based on the mass distribution andthe composition of the object being scanned.

An advantage of the present invention is that the neutrons areessentially mono-energetic. Hence it is possible to model the neutrontransmission using a simple exponential relation and moreover,information is more accurately obtained which is useful for materialidentification.

The equipment according to at least one embodiment of the invention hasthe added advantage of direct measurement of transmitted neutrons and istherefore much more efficient when compared with prior art systems whichmeasure secondary radiation such as neutron-induced gamma rays.

The radiographic equipment may utilise one or more neutron energies. Inan example of a dual neutron energy technique, the radiographicequipment may utilise two tubes, one to produce substantially 14 MeVneutrons via the deuterium-tritium fusion reaction and a second toproduce substantially 2.45 MeV neutrons via the deuterium-deuteriumfusion reaction. The measurement of the neutron transmission at a secondenergy can be used to enhance the capability of the single energytransmission technique.

The source of X-rays or gamma-rays may comprise a radioisotope sourcesuch as ⁶⁰Co or ¹³⁷Cs with energy sufficient to substantially penetratethrough the object to be imaged. The ⁶⁰Co or ¹³⁷CS source may have anenergy of about 1 MeV although other energies may be used depending onthe source. Alternatively an X-ray tube, or an electron linearaccelerator to produce Bremsstahlung radiation could be used.

Collimation of both the source of X-rays or gamma-rays and the source ofneutrons, advantageously acts to minimise scattering. Furthermore,appropriate collimation of both the sources and detector ensures anarrow beam geometry and therefore greater accuracy when determining theattenuation of neutrons and gamma rays through an object. Moreover, thehighly collimated fan-shaped beam provides increased radiation safety.The collimating block may be manufactured from thick paraffin, thickconcrete, iron-shot concrete shielding blocks, steel, lead, or the like.Similarly, the or each detector array may be housed within a detectorshielding having a slot in order to provide the collimation. Thedetector collimation shielding may be made from iron and may have athickness of greater than about 100 mm. The width of the slot may beselected to allow direct passage of neutrons and gamma rays from thesource to the detector and to shield the detector array from scatteredradiation. The detector slot may be about the same width as the detectorarray. The source collimator slots may be narrower.

The detector array may comprise one or more columns of scintillatorpixels.

The same detector array may be able to sense both neutrons and X-rays orgamma rays. Energy discrimination may be used to distinguish the signalsor the detector can operate sequentially on the neutrons and X-rays orgamma rays. An advantage of using the same detector array to senseneutrons and X-rays or gamma rays, is that a reduction in the cost ofthe detector array may be achieved.

Optionally, separate detector arrays may be used to respectively sensethe neutrons and X-rays or gamma rays, with or without separate neutronand X-ray or gamma-ray detector collimators.

The scintillators may be selected such that their spectral response isclosely matched to the photodiodes. The scintillators may further besurrounded by a mask to cover at least a portion of each of thescintillators, each mask having a first reflective surface to reflectescaped light pulses back into the scintillator. The mask will have anopening to allow scintillator light to be detected by the photodiode.The mask may comprise layers of PTFE tape and/or Tyvek paper.Advantageously, the efficiency of plastic scintillators with a mask forneutrons may be greater than 10%. The material surrounding thescintillators acts to ensure that light which escapes the scintillatorsis reflected back to be detected. In an example where each detectorarray includes orange-light emitting plastic scintillators and siliconphotodiodes, the equipment may advantageously have a higher performanceefficiency allowing images to be collected more quickly. Moreover, theequipment may be manufactured at a relatively cheaper cost.

Silicone oil, GE-688 grease, polysiloaxine, optical cement such as EljenEJ-500 cement, or the like may be used to optically couple thephotodiodes to the respective scintillators.

Where the radiographic equipment comprises a single detector array forsensing both neutrons and X-rays or gamma rays, the scintillators may beplastic scintillators or liquid scintillators.

In a further example where the radiographic equipment comprises dualneutron sources and a source of X-rays or gamma-rays, the scintillatorsmay be plastic or liquid scintillators. In this example, thescintillators may be coupled to photomultiplier tubes.

Where the radiographic equipment comprises separate neutron andgamma-ray detector arrays, the neutron scintillators may bepreferentially plastic scintillators or liquid scintillators and thegamma-ray scintillators may be plastic scintillators, liquidscintillators or inorganic scintillators such as caesium iodide, sodiumiodide or bismuth germanate. Alternatively the X-ray or gamma raydetectors may be ionisation chambers.

The radiation receiving face of each scintillator, or the ‘area’ of eachscintillator, corresponds to a single pixel. The area of eachscintillator may typically be smaller than about 20 mm by 20 mm. Smallerareas lead to improved spatial resolution.

The thickness of each scintillator may be in the range 50 to 100 mm andmay be a function of the detection efficiency and light collectionefficiency. In an example where the object to be imaged is a unit loaddevice or ULD such as those typically used in airport environments, theradiation receiving face of the array of scintillators may havedimensions of about 120 mm×3300 mm and may comprise about 1000 pixels.When combined with a 14 MeV neutron source energy of approximately 10¹⁰neutrons/second, the contents of a single ULD may be imaged over a timeperiod of about one minute.

Alternatively separate neutron and gamma-ray scintillators may be used,comprising, for example, about 1000 neutron pixels and about 500 gammaray pixels. In practice the gamma ray pixels may be made smaller thanthe neutron pixels which advantageously provides high-resolution spatialimages.

In a further example, the conversion means may comprise photomultipliertubes and wavelength shifting optical fibres (WSF). In this example,light from a row or column of scintillator rods may be collected by theWSF and transmitted to a multi-anode photomultiplier tube. By indexingthe row and column producing the light pulse, the scintillator rodintercepting the radiation maybe inferred.

The conversion means may include low noise and high gain amplifiers toamplify the output signals. The conversion means may include a computerto perform image processing and display the images to an operator on acomputer screen.

The detector may be temperature controlled to reduce noise and improvestability. For instance, the photodiodes and preamplifiers may be cooledto about −10° C. or lower.

In one example, as an object to be imaged is scanned, one or moreoutputs are obtained measuring the transmission of, for instance, the 14MeV neutrons through the object and the transmission of the 1 MeV X-rayor gamma-rays through the object. For dual energy neutron scanning, thetransmission of say the 2.45 MeV neutrons through the object is alsomeasured. The invention is not limited to the use of these energiesalone.

Where a single detector array is used for receiving radiation energyfrom the source of X-rays or gamma rays and the source of neutrons, theobject may be scanned more than once.

Where separate detectors are used for receiving radiation energy fromthe source of neutrons and the source of X-rays or gamma rays, theoutput signal may comprises a first output from the first array ofscintillators and a second output from the second array ofscintillators, where the first output is related to the neutron countrate in each pixel location of the detector, and the second output isrelated to the X-ray or gamma-ray count rate in each pixel location ofthe detector.

Each of the source inputs may be separately processed. A simplescintillation spectrum may be collected separately for each pixel of thearray to deduce neutron and X-ray or gamma-ray count rates for eachpixel. The information may then be assembled to form a complete2-dimensional neutron image and a complete 2-dimensional X-ray orgamma-ray image. The resulting image may have a vertical resolutiongoverned by the pixel size, and a horizontal resolution governed by thepixel size and the frequency with which the array is read out.

The computer may also be able to perform automatic materialidentification. For instance, the transmission outputs may be convertedto mass-attenuation coefficient images for each pixel for display on acomputer screen with different pixel values mapped to different colours.In particular mass-attenuation coefficient images may be obtained fromthe count rates measured from the transmissions for each of the 14 MeVneutrons and X- or gamma-rays or the 14 MeV neutrons, 2.45 MeV neutronsand X- or gamma-rays.

Analysis of the mass-attenuation coefficient images allows a variety ofinorganic and organic materials to be distinguished. Such analysis mayinclude forming cross section ratio images between pairs of massattenuation coefficient images. Depending on whether a single or dualneutron sources are utilised, cross section ratio images may be formedfrom the mass-attenuation coefficient images of the source of neutronsand the X-rays or gamma-rays, or the first and second sources ofneutrons and the first or second source of neutrons and the X-rays orgamma-rays. For example, the 14 MeV neutrons and the X-rays orgamma-rays, the 14 MeV neutrons and the 2.45 MeV neutrons, and the 2.45MeV neutrons and the X-rays or gamma-rays. Advantageously, such ratiosare independent of the mass of the object.

The proportions in which the cross section ratio images are combined maybe operator adjusted to maximise contrast and sensitivity to aparticular object being examined in the image.

An image may be formed that is a linear combination of two cross sectionratio images.

Two regions in an image may be identified which contain a firstsubstance, but only one of the regions may contain a second substance.By performing cross section subtractions the image of the firstsubstance may be effectively removed leaving the image of the secondsubstance available for identification. The mass of the second substancemay be obtained from the X- or gamma-ray transmission data.

In one example, the source of neutrons and the detector are stationaryand the conveyance means is arranged such that the object is moved infront of the source of neutrons and gamma rays. In a further example,the object may be stationary and the conveyance means arranged such thatthe source and the detector move in synchronicity either side of theobject. In a still further example, multiple sets of detectors may besituated around sources which are centrally located to allow scans of aplurality of separate objects to be acquired simultaneously. This wouldhave the advantage of improving throughput. In such an example, theconveyance means may be arranged such that the objects can be movedbetween the source of neutrons and the respective detector.Alternatively, the sources and detectors can be rotated around theobject to be examined to allow multiple views to be obtained.

The rate at which the object is able to be moved in front of either thesource of neutrons, or, the source of neutrons and X-rays or gamma-raysis partially dependent on the intensity of the neutron and gamma raysources. The intensity of the single neutron source of 14 MeV may be inthe order 10¹⁰ neutrons/second, or as high as practically possible inorder to improve counting statistics.

The rate at which the object is able to be moved in front of the sourceof neutrons and X-rays or gamma-rays is further dependent on theradiation receiving face of the array of scintillators and the number ofscintillators. In addition, the length of the array is partiallydependent on the length of the object to be imaged.

The object may be scanned between the neutron and gamma ray sources anddetector and may pass through a shielded tunnel. The conveyance meansmay comprise a pair of rails for the positioning of a dolly or platformon which the object may be transported. Alternatively, the conveyancemeans may include a conveyor belt or other like arrangement for passingor winching objects through the tunnel. The conveyance means may beautomated such that the object is smoothly transported in front of thesource of neutrons at a controllable uniform rate.

The invention may be applied to non-invasive examination of sea cargo,air cargo Unit Load Devices (ULD), or smaller containers or packages,the detection of contraband, explosives and other articles, substancesand materials. It may provide improved specificity for contrabandmaterials, such as organic materials in primarily inorganic matrices, aswell as the detection and identification of specific classes of organicmaterial. It is particularly suited for the detection of explosives,narcotics and other contraband items concealed in aircraft baggage,airfreight containers and shipping containers.

A further advantage of at least one embodiment of the invention is thatuse of a neutron generator for producing neutrons is able to be switchedon and off.

It may also provide increased automation of the inspection process, withreduced reliance on human operators.

Further, it may provide a fast scanning rate so that a high throughputcan be achieved. It is simple, low-cost and uses safe radiation sources;and simple, low-cost radiation detection systems. It may operate with ahigh detection rate and low false alarm probability.

BRIEF DESCRIPTION OF DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

Several examples of the invention will now be described with referenceto the accompanying drawings, in which:

FIG. 1 a is a perspective view of an example of the radiographicequipment;

FIG. 1 b is a perspective view of a portion of the radiographicequipment shown in FIG. 1 a;

FIG. 1 c is a sectional view along line A-A of FIG. 1 b;

FIG. 2 a is a schematic illustration of one module of the radiographicequipment's detector array;

FIG. 2 b is an end view of a scintillator illustrated in the detectorarray shown in FIG. 2 a, where the scintillator is surrounded by a maskhaving a first reflective surface;

FIG. 2 c is a schematic illustration of a further variation of theradiographic equipment's detector array:

FIG. 3 is a bar graph of the calculated ratio, R, the ratio of the 14MeV neutron to ⁶⁰Co gamma-ray mass attenuation coefficients for a largenumber of benign, narcotic and explosive materials;

FIG. 4 is a plot of the calculated ratio, R, the ratio of the 14 MeVneutron to the ⁶⁰Co gamma-ray mass attenuation coefficients for a rangeof elements;

FIG. 5 a is a display output of a gamma-ray scan of a motor bike, FIG. 5b is a display output in which the image is coloured according to themass attenuation coefficient ratio, R, for 14 MeV neutrons and gammarays;

FIG. 6 a is a schematic illustration of a selection of material samplesand common objects arranged on wooden shelves; FIG. 6 b is a displayoutput of a gamma-ray scan; FIG. 6 c is a display output in which theimage is coloured according to the mass attenuation coefficient ratio,R, for 14 MeV neutrons and gamma rays;

FIG. 7 a is a schematic illustration of a selection of material samples,concealed contraband, alcohol, as well as simulated and real explosives;FIG. 7 b is a display output of a gamma-ray scan; FIG. 7 c is a displayoutput in which the image is coloured according to the mass attenuationcoefficient ratio, R, for 14 MeV neutrons and gamma rays;

FIG. 8 a is a photograph of a ULD containing assorted householdelectronics metal items, concrete blocks and concealed contraband; FIG.8 b is a display output of a gamma-ray scan; FIG. 8 c is a displayoutput in which the image is coloured according to the mass attenuationcoefficient ratio, R, for 14 MeV neutrons and gamma rays; FIG. 8 d isthe display output of figure 8 c which has been further processed toemphasise the organic material;

FIG. 9 a is a photograph of a ULD containing assorted household itemsand concealed drugs; FIG. 9 b is a display output of a gamma-ray scan;FIG. 9 c is a display output in which the image is coloured according tothe mass attenuation coefficient ratio, R, for 14 MeV neutrons and gammarays R; FIG. 9 d is the display output of FIG. 9 c which has beenfurther processed to emphasise the organic material;

FIG. 10 a is a photograph of a ULD containing assorted household itemsand concealed drugs, FIG. 10 b is a display output of a gamma-ray scan;FIG. 10 c is a display output in which the image is coloured accordingto the mass attenuation coefficient ratio, R, for 14 MeV neutrons andgamma rays; FIG. 10 d is the display output of figure 10 c which hasbeen further processed to emphasise the organic material;

FIG. 11 is a plot of a large number of benign, narcotic and explosivematerials in terms of two cross-section ratios, namely 2.45 MeVneutron/14 MeV neutron cross-sections versus 14 MeV neutron/X- orgamma-ray cross-sections;

FIG. 12 a is a simulated count rate DT neutron image of a suitcase; FIG.12 b is a simulated count rate image of a DD neutron image of thesuitcase; FIG. 12 c is a simulated count rate X ray image of thesuitcase; FIG. 12 d is a DT/X-ray cross section image and FIG. 12 e is aDD/DT cross-section image;

FIG. 13 a is a simulated 14 MeV neutron image of an air freightcontainer; FIG. 13 b is an X-ray image respectively of the samecontainer; and FIG. 13 c is a combined image of the same container; and

FIG. 14 is a perspective view of a further example of the radiographicequipment.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 a illustrates the general layout of the radiographic equipment10. The equipment 10 includes two separate generators of radiation, thefirst is an A-325 MF Physics neutron generator having a D-T neutronemitting module to produce a neutron energy source 12 having an energyof 14 MeV. The neutron generator is operated at a voltage of 80-110 kV.The second generator of radiation is a 0.82 GBq (or 22 mCi) ⁶⁰Co source14 to produce a source of gamma-rays and is located to the right of andadjacent to the neutron generator. The neutron generator and ⁶⁰Co source14 are situated within a collimating block in the form of a sourceshield housing 16.

A 1600 mm long and 20 mm wide detector array 18 is situated in thevicinity of the radiation source and is housed in a detector shieldhousing 20. The detector array 18, more clearly shown in FIG. 2 a isbuilt up of eighty plastic scintillator rods 19 (of which only a portionare shown), each with a radiation receiving area of 20 mm×20 mm, and alength of 75 mm. The radiation receiving area of each scintillator rod19 corresponds to a single pixel in the image-frame. The term imageframe is used to describe the two-dimensional array containing thenumber of counts measured in each pixel, accumulated over a fixed timeinterval. The scintillator rods 19 are made of an orange plasticscintillator in order to match the spectral response of the siliconphotodiodes 21 to the respective plastic scintillators. The photodiodes21 are optically coupled to respective scintillators 19 with opticalcement. A reflective mask 31 (of which only one is illustrated) ispainted on each of the orange scintillator rod and photodiodecombinations to minimise the loss of any light that escapes thescintillator rods. As illustrated in FIG. 2 b, each mask 31 has a firstreflective surface 33 to reflect escaped light pulses back into thescintillator 19.

In the primary embodiment, the scintillation light produced in a rod 19by an incident neutron or X- or gamma-ray is detected by a photodiode 21attached to the end of the rod 19. In a first variation, light from arow or column of scintillator rods is collected by a wavelength shiftingoptical fibre and transmitted to the photodiode. By indexing the row andcolumn producing the light pulse, the scintillator rod intercepting theradiation can be inferred. In a second variation, illustrated in FIG. 2c, light from several rows or columns of scintillator rods 190 iscollected by wave-length shifting optical fibres 40 and transmitted to aposition sensitive photodiode or multi-anode photomultiplier 42. Byindexing the row and column producing the light pulse, the scintillatorrod intercepting the radiation can be inferred.

Since respective photodiodes 21 have no internal gain, the signalconditioning electronics 23 include preamplifiers used in conjunctionwith high-gain amplifiers in order to amplify the output signal for bothneutrons and gamma-rays.

With reference to FIG. 1 a, a computer 15 is provided to generate outputrepresenting the mass distribution and composition of the objectinterposed between the sources 12, 14 and detector array 18. A displayscreen 25 is further provided for displaying images based on the massdistribution and the composition of the ULD 28 being scanned.

The equipment 10 accommodates a ULD 28 with a width up to 2.5 m and aheight of 1.7 m. Each ULD 28 to be imaged is mounted on a platform 30that has runners to engage with a pair of tracks 32. In practice in anairport the ULDs could be scanned while still mounted on theirrespective dollies that are used to transport the ULDs around theairport. The ULDs and their dollies could be driven onto a platform thatwould traverse the radiation beams at a known speed. This would minimisethe handling of ULDs at the airport.

A further shield in the form of a tunnel 34 is provided. The tunnel 34is sufficiently long enough so that the equipment can be operatedwithout doors on either end. This permits the number of ULDs passingthrough the equipment 10 to be maximised.

With reference to FIGS. 1 b and 1 c, collimating slits 38, 39 are cutinto the source and detector shield respectively 16, 20, serve to definea fan shaped radiation beam 17, directed from the sources 12 and 14towards the radiation detector 18. The detector collimating slit 39 anddetector 18 extend the full height of the tunnel 34. Slots (not shown)in the sides of the shield 34 are provided and mate with collimatingslits and for the passage of radiation from the sources 12, 14 to thedetector 18.

Each of the radiation shields 16, 20 and 34, attenuate and absorb bothgamma rays and neutrons. Shielding materials used include concrete, ironand polyethylene. The radiation shields 16, 20 and 34 provideradiological protection for operators of the equipment or other personsin its immediate vicinity.

In operation, objects that are to be imaged are situated on the platform30 that is then motorised through the tunnel 34. In the full-scaleprototype scanner described here the platform 30 is typically operatedat a speed such that each 10 mm increment takes approximately fortyseconds to collect. This corresponds to a speed of 0.25 mm/sec;consequently, about 2½ hours are required to collect the image of a fullULD. In practice, the speed at which the ULD travels through theequipment can be increased by a factor of over one hundred by increasingthe intensity of the neutron source and by increasing the area of thedetector array.

As the object passes through the tunnel 34, a scintillation spectrum iscollected separately for each element of the 80-pixel array. Thesespectra are read out and reset every time the platform 30 traverses 10mm and the spectra are used to deduce neutron and gamma-ray count ratesfor each pixel. The information in each vertical strip is then assembledto form complete, 2-dimensional neutron and gamma-ray images.

The resulting image has a vertical resolution of 20 mm, governed by thepixel size, and a horizontal resolution of 10 mm, governed by thefrequency with which the 80-pixel array is read out. As discussed below,deconvolution of the final image is performed to correct any blurringthat may arise as a result of the combination of the motion of theplatform 30 during the scan and the 20 mm width of the pixels.

Suppose that the neutron intensity and gamma-ray intensity transmittedthough an object and detected in a particular pixel from each image areI_(n) and I_(g) respectively and that the neutron intensity andgamma-ray intensity transmitted and detected in a particular pixel fromeach image without an object present are I_(on) and I_(og) respectively.

Then the attenuation of essentially monoenergetic fast neutrons throughan object of density ρ and thickness x can be calculated using theequation:I _(n) /I _(on)=exp(−μ₁₄ ρx)  (1)

Similarly the attenuation of essentially monoenergetic gamma rayattenuation through the object can be written as:I _(g) /I _(og)=exp(−μ_(g) ρx)  (2)where μ₁₄ is the neutron mass attenuation coefficient at 14 MeV andμ_(g) is the gamma mass attenuation coefficient. The mass attenuationcoefficient ratio can then be calculated directly:R=μ ₁₄/μ_(g)=ln(I _(n) /I _(on))/ln(I _(g) /I _(og))  (3)

Where R is directly related to the composition of the object and allowsa wide variety of inorganic and organic materials and elements to bedistinguished.

FIGS. 3 and 4 illustrate the ability of R to distinguish a wide varietyof inorganic and organic materials. Natural materials that are primarilycarbohydrate based such as cotton, paper, wood as well as many foods,protein based natural materials such as wool, silk and leather andsynthetic organic materials—mainly polymers can be broadlydistinguished. As illustrated, inorganic materials such as pottery,ceramics and metal items are easily distinguished from organicmaterials.

Due to the higher count-rates and lower background scattering of thegamma rays, the gamma-ray image carries most of the information aboutshape and density. For each pixel in the image, the quantityln(I_(g)/I_(og)) is calculated, which is proportional to the total massper unit area of material along the line from the radiation source tothe pixel in question. A “Mexican-hat” sharpening filter is applied tothis image to improve object definition and reduce the effects of themotion and pixel-size blurring that affects the horizontal resolution ofthe image.

The pixel-by-pixel ratio of the neutron and gamma-ray images carriesinformation about the average composition of each pixel, which isindependent of the amount of intervening material.

Due to the relatively low counting statistics in the neutron image,there is considerable pixel-to-pixel noise present in the compositionimage. Consequently, a 5×5-pixel Gaussian smoothing filter is applied tothis image. Whilst this reduces the resolution of the compositioninformation in the final image, it significantly enhances the visibilityof subtle changes in composition for objects with dimensions of morethan about 50 mm.

The results from six scans are shown in FIGS. 5 to 10. The gray-scaleimages illustrate the results of the gamma-ray scan alone and as suchshow the results that would be achievable from a conventional X-rayscanner. Regions with little or no intervening material show as whiteand denser materials show as darker shades of grey. The colour imagescombine the gamma-ray shape and density information, together with thecomposition information from the neutron/gamma ratio image. The densityof colour shows the material density with white corresponding to nointervening material and denser regions having a saturated colour. Thecolour of a pixel corresponds to the R value for that pixel, with lowerR values coloured blue, intermediate values turquoise through green toyellow and higher values orange. The exact mapping between R value andcolour is different for each image, with the colour scale adjusted toshow the maximum information in each case. For the ULD scans, anenhanced organic image is also presented. This emphasises organicregions of the image, which are coloured yellow, orange and red.

FIG. 5 a illustrates the result of the gamma ray scan alone of amotorbike. FIG. 5 b illustrates the combined gamma-ray shape and densityinformation together with the composition information from theneutron/gamma ratio image scan of a motorbike. This image provides agood indication of the overall imaging capabilities of the equipment. Inparticular, fine details such as the front brake cables 52 show quiteclearly in FIG. 5 b, even though they are considerably smaller than the20 mm pixel size. The metal frame 54 and engine 56 of the bike show upblue in figure 5 b; whereas the fuel 58 in the petrol tank, rubber tyres60, plastic seat 62 and plastic lights show up orange. The oil 64 in thesump (immediately above the kickstand), when averaged together with themetal around it shows as a green patch. In contrast, from theconventional gamma-ray image FIG. 5 a, it is difficult or impossible todistinguish between the oil 64 and the sump.

FIGS. 6 a to 6 c illustrate a selection of material samples and commonobjects arranged on wooden shelves. Again, as illustrated in FIG. 6 c,metals such as iron 66, lead 68 and aluminium 70 show up dark-blue.Intermediate materials such as concrete 72, glass 74 (in the computermonitor 75) and ceramic powder (alumina, Al₂O₃) 76 show up lighter blue.Finally, the organic materials, including elemental simulants of heroin77, methamphetamine 78, cocaine 80 and TNT 82 show up in a variety ofcolours from green to orange, depending on the R value of the material.Two ceramic statues on the top shelf, one filled with iron shot 84 andthe other with sugar 86 can be clearly distinguished, both by densityand by composition.

FIG. 7 a to 7 c illustrate a further selection of materials, includingconcealed contraband, alcohol and both simulated and real (Detasheet)explosives. Three hollow concrete blocks are positioned on the topshelf. The left-hand block contains concealed organic material 94 (drugsubstitute); the centre block is empty and the right hand block containsalumina powder 96. These three blocks provide simple models of drugsconcealed within a ceramic or pottery object, a hollow, empty object anda hollow, empty object with thickened walls. Whilst the gamma-ray imageof FIG. 7 b clearly distinguishes between the empty 95 and filled blocks94 and 96, it cannot separate the drug-surrogate filled block 94 fromthe alumina filled block 96. In contrast, the neutron image of FIG. 6 cclearly reveals the concealed organic filling 94 shown as ayellow/orange patch. On the left hand side of the middle shelf arepositioned two containers, one filled with pure alcohol 98 (Meths) andone with water 100 (H₂O). The alcohol 98 shows clearly as being more‘organic’ (higher R value) and is predominantly orange in colour; thewater 100, with a lower R value is predominantly green. On the sameshelf, the simulated 102 and real 104 explosives show as the same colourshowing that the simulant is a good substitute for real explosive. Onthe bottom shelf is a case containing twelve glass bottles of which onlyfour are visible, two filled with simulated spirits 106 (40% ethanol,60% water) and two filled with water 108. Again, the alcohol filledbottles 106 show up as having a higher R value (more green/orange) thanthe water 108 (predominantly blue). This is in contrast to the bottlesshown in FIG. 7 b which are almost indistinguishable.

FIGS. 8 a to 8 d, 9 a to 9 d and 10 a to 10 d illustrate the results ofimaging ULDs filled with a variety of objects. In all three figures, thefilling of the ULD has been deliberately kept fairly simple, to simplifydiscussion of the results obtained. In particular, most of the packingmaterial that would normally be present (cardboard boxes, foam,polystyrene etc) has been omitted so that the objects in the ULD can beclearly seen. It is recognised that in reality, most ULDs would beconsiderably more cluttered.

FIGS. 8 a to 8 d illustrates a ULD filled with a variety of householdelectronics (a refrigerator 120 and several computers 122), metal parts,hollow concrete blocks 124 (substituting for ceramic pipes or hollowstatues or figurines) and tools. Two packets of plastic beads,substituting for drugs 126, are concealed within one of the computersand inside one of the concrete blocks. A propane gas cylinder 128 isalso hidden inside the ULD. FIG. 8 a illustrates a photograph of the ULDscanner. FIG. 8 b shows the results of the gamma-ray scan only. Neitherof the packets of surrogate drugs 126 are particularly obvious. Thepropane gas cylinder 128 can be identified on the basis of its shape,although the organic nature of its contents is not clear. FIGS. 8 c and8 d are coloured according to the neutron/gamma ratio R, as a result theinorganic materials show up in FIG. 8 c as blue (the computer 122 andblocks 124) and the organic materials as orange (the surrogate drugs 126and the gas cylinder 128). The proportions in which the two images arecombined are adjusted by the operator to maximise contrast andsensitivity for organic materials which are coloured yellow and red andto minimise the effects of clutter resulting from overlapping objects,the result is illustrate in FIG. 8 d. Clearly both packets of concealeddrugs 126 can be identified.

FIGS. 9 a to 9 d illustrates a ULD with drugs 124 concealed inside twocomputers 122 and a fridge 120. Whilst it can be seen in the gamma-rayimage of FIG. 8 b that the top two computers 122 appear somewhatdifferent from the bottom two, it is not clear whether this is a genuinedifference in the structure of the machines. However, in the FIGS. of 9c and 9 d it is immediately apparent that the difference is due to alarge volume of organic material, as shown by the bright orange colourof these regions with drugs 124. The top two computers 122 contain ˜1 kgbags of plastic beads simulating packaged drugs. This is in contrast tothe predominantly blue (inorganic or low R value) colour of the rest ofthe computer structure 126. Similarly, it is not clear from thegamma-ray image of FIG. 9 b of the fridge 120 whether the anomaly in thecentre of the image is part of the structure of the fridge or not.However, in FIGS. 9 c and 9 d it can be seen that the anomaly 124 isclearly organic and in contrast to the predominantly inorganic structurevisible in the rest of the fridge (in particular, the compressor 125 atthe lower right and the freezer compartment at the top). Again, in theenhanced organic image of FIG. 9 d the concealed drugs 124 are clearlyvisible. Additionally, other organic material in the ULD (notably thewooden shelving 128 behind the fridge 120 and the container of water 127to the left of the fridge 120) also shows up as orange.

FIGS. 10 a to 10 d illustrate a second ULD with real concealed drugs (1kg each of heroin and methamphetamine). The heroin 130 is hidden insidea hollow concrete block 132. The methamphetamine 134 is hidden inside asmall box, which is placed inside a larger box 136 filled with clothing.The organic nature of the concealed drugs is evident from the colouringin the composition images of FIGS. 10 c and 10 d. Once again, theenhanced organic image of FIG. 10 d effectively reveals the concealeddrugs 130 and 134, especially the heroin 130 coloured yellow inside theconcrete blocks 132. As the methamphetamine 134 is concealed within thebox 136 of clothing (immediately behind the front fork of the bicycle140), composition discrimination is less revealing in this case.However, the package of drugs 134 can be identified as a potentialanomaly on the basis of its shape and higher density.

The radiographic equipment as described can be used in at least threeways for detecting and identifying contraband materials. Firstly, thegamma-ray images provide considerable information about the shapes,sizes and densities of objects inside an object such as a ULD. Somesuspicious materials can be identified on this basis. Particularexamples would be packets of drugs concealed inside spaces or cavitiesof hollow objects. Secondly, the colouring of the gamma-ray image on thebasis of composition information derived from the neutron measurementsprovides powerful extra clues in the interpretation of scan images andidentification of suspicious materials. In particular, the detection oforganic materials inside predominantly inorganic objects is greatlyfacilitated. Thirdly, under certain circumstances, the equipment can beused to measure the neutron/gamma ratio (R values) of suspiciousmaterials to further assist in their identification. This approach worksbest when there is little over- or under-lying material around thesubstance being measured, or when the over- and under-lying material isreasonably uniform in the immediate vicinity of the measurement region.Under these circumstances, it is possible to make an approximatecorrection for the absorption of neutrons and gamma rays in the over-and under-lying material to obtain the R value of just the substance ofinterest.

A second embodiment applies directly to the dual energy fast neutrontransmission embodiment for 14 MeV and 2.45 MeV. However the followingdiscussion also applies to the dual energy transmission at differentenergies to 2.45 and 14 MeV. However unlike single energy neutrontransmission discussed previously, three count rates are measured ateach pixel rather than two in the case of single neutron transmission,and two-cross-section ratios can be calculated.

Suppose that the count rates in a particular pixel from each image arer₁₄, r_(2,45) and r_(x) respectively. These rates are related to the(unknown) mass of material m between the source and detection points andthe (unknown) mass attenuation coefficients of this material for 14 MeVneutrons, 2.45 MeV neutrons and X- or gamma-rays, written as μ₁₄,μ_(2.45) and μ_(X) respectively, by the relations:r ₁₄ =R ₁₄exp(−mμ ₁₄)  (4)r _(X) =R _(X)exp(−mμ _(X))  (5)r _(2.45) =R _(2.45)exp(−mμ _(2.45))  (6)

where R₁₄, R_(2.45) and R_(X) are respectively the count rates for 14MeV neutrons, 2.45 MeV neutrons and X- or gamma-rays when no interveningobject is present.

The cross-section ratios can be calculated directly:μ₁₄/μ_(X)=log(r ₁₄ /R ₁₄)log(r _(X) /R _(X))  (7)μ_(2.45)/μ₁₄=log(r _(2.45) /R _(2.45))log(r ₁₄ /R ₁₄)  (8)

Note that both of these ratios are independent of the mass of materialpresent in the beam between the source and detector.

The cross-section ratios given by equations (7) and (8) allow a widevariety of organic and inorganic materials to be distinguished.

FIG. 11 illustrates the ratio of 2.45 MeV neutron cross-section to 14MeV neutron cross-section versus the ratio of 14 MeV neutroncross-section to X- or gamma-ray cross-section, for a selection ofmaterials. The availability of two cross-section ratios further enhancesthe ability of the invention to distinguish between different materials.Consequently, analysis of the three mass-attenuation coefficient imagesallows information about the contents of the object being examined to beinferred.

FIG. 12 illustrates the additional benefit of using dual neutronenergies, consider the simulated images of a suitcase 150 shown in FIGS.12 a to 12 e. Images 12 a to 12 c correspond to equations (4) (5) and(6) and show the transmission of 14 MeV neutrons, 2.45 MeV neutrons andX- or gamma-rays respectively. Images 12 d to 12 e correspond toequations (7) and (8) and show the DT/X-ray and DD/DT cross-sectionsrespectively.

The suitcase 150 is filled with clothing composed of cotton and wool,and contains various benign and suspicious objects. Bottle 152 containswater and bottle 154 contains spirits. The three blocks visible on thelower right of the suitcase 150 are a paperback book 156, heroin 158 andRDX explosive 160. A gun 162 is also visible in the upper right of thesuitcase 150.

From a conventional X-ray image 12 c, it is difficult or impossible todistinguish between the contents of the two bottles 152, 154, or thethree packages 156, 158, 160 on the right hand side of the case thathave similar densities. The neutron images 12 a, 12 b provide morecontrast between the different materials, but the best results areobtained from the cross-section ratio images 12 d and 12 e. Inparticular, the book 156 as shown in FIGS. 12 a and 12 b virtuallydisappears in FIGS. 12 d and 12 e as paper has a similar composition tothe surrounding clothing, whereas the drugs 158 in FIG. 12 e andexplosive materials 160 in FIGS. 12 d and 12 e can be clearlydistinguished. A clear difference is also seen in both FIGS. 12 d and 12e between the bottles containing water 152 and spirits 154.

In a first variation of the dual neutron transmission method, theoperator would form a new image that is a linear combination of the twocross-section ratio images. The proportions in which the two images arecombined are adjusted by the operator to maximise contrast andsensitivity for contraband materials and to minimise the effects ofclutter resulting from overlapping objects.

FIGS. 13 a to 13 b illustrate simulated 14 MeV neutron and X-ray imagesrespectively of a container 170, taken from the side. Due to their highdensity, the steel pipes 176 dominate the images, making it hard to seethe outlines of the computer equipment. However, by forming a singleimage, FIG. 13 c, from the two cross-section ratio images given byequations (7) and (8), it is possible to remove the “clutter” associatedwith the steel pipes 176, to reveal the computer boxes 174.

This approach can be understood with reference to FIG. 11. Choosing alinear combination of images (7) and (8) is equivalent to colouringimage pixels according to their distance from an arbitrarily orientatedline drawn on FIG. 11. By choosing this line to be parallel to twoselected materials, any combination of these materials is coloured thesame. In the example discussed, the line is chosen to be parallel to aline connecting steel and the polystyrene packaging of the computers. Inthis way, the steel pipes can be made to largely vanish where they passin front of the computers. FIG. 13 c shows the results of this process.

Although one such example of the invention has been discussed, it shouldbe appreciated that such an embodiment is only one of the many utilisingthe principles of the invention. Whilst in the above example, theradiation sources are situated on one side of the object to be examinedand the detectors on the opposite side, in a first variation, thesources are situated above or below the object to be examined, with thedetectors positioned on the opposite side (below or above respectively).In a second variation, the sources and detectors can be rotated aroundthe object to be examined to allow multiple views to be obtained. In athird variation, multiple sets of sources and detectors are used toallow simultaneous collection of multiple views of the same object. In afourth variation, multiple sets of detectors are disposed around acentral source to allow views of multiple objects to be acquiredsimultaneously.

Of course, in operation, objects that are to be scanned may be passedthrough the tunnel on a conveyor belt or winched or pushed through usinga suitable mechanism.

Whilst in the above embodiment, the two radiation sources are operatedsequentially as the object is scanned through the analyser. In a firstvariation, the object is scanned through the analyser twice, with onesource being operated for each scan. In a second variation, each sourcehas a separate associated detector and the object is scanned only once.In a third variation, the two radiation sources are operated at the sametime, a single detector is used and energy discrimination is used toseparate the signals due to neutron and X- or gamma-rays.

In the variation (dual neutron energy embodiment 100) as illustrated inFIG. 14, the radiation source comprises three separate generators ofradiation, one producing 14 MeV neutrons 112, one producing 2.45 MeVneutrons 113, and the last producing high-energy X- or gamma-rayradiations 114. The neutron sources are sealed tube neutron generatorsor other compact sources of a similar nature, producing neutrons via D-Tand D-D fusion reactions.

The three radiation sources are operated sequentially as the object isscanned through the analyser. In a first variation, the object isscanned through the analyser three times, with one source being operatedfor each scan. In a second variation, each source has a separateassociated detector (desianated generally as 118) and the object isscanned only once. In a third variation, two or more of the radiationsources are operated at the same time with a single detector, and energydiscrimination is used to distinguish the signals from the high energyneutrons, low energy neutrons and X- or gamma-rays.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive.

References

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1. Radiographic equipment comprising: a first neutron source ofsubstantially mono-energetic fast neutrons produced via thedeuterium-tritium or deuterium-deuterium fusion reactions, comprising asealed-tube generator for producing the neutrons; a source of X-rays orgamma-rays of sufficient energy to substantially penetrate an object tobe imaged, the source of X-rays or gamma-rays being physically separatedfrom the first neutron source; a collimating block surrounding theneutron source and the X-ray or gamma-ray source, and comprising one ormore slots for emitting substantially fan-shaped radiation beams; adetector array comprising a multiplicity of individual scintillatorpixels to receive neutron radiation and X-ray or gamma-ray radiationemitted from the respective sources and to convert the receivedradiation into light pulses, the detector array aligned with thefan-shaped radiation beams emitted from the source collimator andcollimated to substantially prevent radiation other than that directlytransmitted from each of the sources from reaching the array; convertorfor converting the light pulses produced in the scintillators intoelectrical signals; conveyor for conveying the object between each ofthe sources and the detector array; computing device for determiningfrom the electrical signals the attenuation of the neutrons and theX-ray or gamma-ray beams and to generate output representing the massdistribution and composition of the object interposed between each ofthe sources and detector array; and display for displaying images basedon the mass distribution and the composition of the object beingscanned.
 2. Radiographic equipment according to claim 1, where the X-rayor gamma-ray source comprises a ¹³⁷Cs, ⁶⁰Co or similar radioisotopesource having an energy of substantially 1 MeV.
 3. Radiographicequipment according to claim 1, where the X-ray or gamma-ray sourcecomprises an X-ray tube or electron accelerator producing X-rays throughBremsstrahlung on a target.
 4. Radiographic equipment according to claim1, where the neutron source produces neutrons having substantiallyhigher energies than the X-ray or gamma-rays from the X-ray or gamma-raysource, where the neutron and X-ray or gamma-ray sources are arranged topass through the same slot in the collimating block and a singledetector array is used, comprising individual pixels of plastic orliquid organic scintillator, where discrimination between the X-rays orgamma-rays and the neutrons is made on the basis of the energy theydeposit in the scintillator.
 5. Radiographic equipment according toclaim 1, where the sources of neutrons and X-ray or gamma-rays arearranged to pass through the same slot in the collimating block and asingle detector array is used comprising individual pixels of plastic orliquid organic scintillator, where the neutron and X-ray or gamma-raysources are operated alternately.
 6. Radiographic equipment according toclaim 1, where the sources of neutrons and X-ray or gamma-rays arearranged to pass through separate parallel slots in the collimator blockand two detector arrays are used, one comprising individual pixels ofplastic or liquid organic scintillator for the detector of the neutronsand one comprising individual pixels of plastic, liquid or inorganicscintillator for detection of the X-rays or gamma-rays.
 7. Radiographicequipment according to claim 4, where each slot of the source anddetector collimators are sufficiently wide to ensure full illuminationof the detectors by the source, whilst minimising the detection ofscattered radiation.
 8. Radiographic equipment according to claim 1,further comprising a second sealed tube or similar neutron generator forproducing neutrons via either the deuterium-tritium ordeuterium-deuterium fusion reactions, where the second source ofneutrons uses a complementary fusion reaction to the first neutronsource.
 9. Radiographic equipment according to claim 8, where theneutrons from the second neutron source are detected in a separatecollimated detector array comprising individual pixels of plastic orliquid organic scintillator.
 10. Radiographic equipment according toclaim 9, where one of the first neutron source or the second neutronsource has an energy of substantially 14 MeV and the other has an energyof substantially 2.45 MeV.
 11. Radiographic equipment according to claim1, where the convertor comprises a plurality of photodiodes, wherein thescintillator material is selectable to have an emission wavelengthsubstantially matched to the response of the photodiodes. 12.Radiographic equipment according to claim 1, where the convertorcomprises crossed wavelength shifting fibres coupled to a multiplicityof single or multi-anode photomultiplier tubes.
 13. Radiographicequipment according to claim 11, where the electrical signals from theconvertor indicate the transmission of the first neutron source and theX-rays or gamma-rays through the object being scanned, or thetransmission of the neutrons from the first neutron source, the X-raysor gamma-rays and the neutrons from a second neutron source through theobject being scanned.
 14. Radiographic equipment according to claim 13,where mass attenuation coefficient images for each pixel are computedbased on the respective transmissions and displayed with different pixelvalues mapped to different colours, where the image is indicative of themass distribution and composition inferred from the computations. 15.Radiographic equipment according to claim 1, where the computing devicecomprises a computer to perform image processing and display the imageson a computer screen.
 16. Radiographic equipment according to claim 15,where the output is convertable to mass-attenuation coefficient imagesfor each pixel for display on a computer screen with different pixelvalues mapped to different colours.
 17. Radiographic equipment accordingto claim 16, where the mass-attenuation coefficient images areobtainable from count rates measured from the transmissions for each ofthe deuterium-tritium neutrons or deuterium-deuterium neutrons andX-rays or gamma-rays, or the deuterium-tritium neutrons,deuterium-deuterium neutrons and X-rays or gamma-rays.
 18. Radiographicequipment according to claim 17, where the computer is operable toobtain cross section ratio images between pairs of mass attenuationcoefficient images.
 19. Radiographic equipment according to claim 18,where the proportions in which the cross section ratio images arecombined are adjustable to maximise contrast and sensitivity to aparticular object being examined in the image.
 20. Radiographicequipment according to claim 18, where the computer is able to performautomatic material identification based on the measured cross sections.21. Radiographic equipment according to claim 19, where the proportionsin which the cross section ratio images are combined are operatoradjustable.
 22. Radiographic equipment according to claim 1, where thesources and the detector array are stationary and the conveyor isarranged such that the object is able to be moved in front of the sourceof neutrons.
 23. Radiographic equipment according to claim 1, where theobject is stationary and the conveyor is arranged such that the sourceand the detector array move in synchronicity on either side of theobject.
 24. Radiographic equipment according to claim 1, where multipleviews are obtained by either rotating the object relative to the sourcesand the detector array or by rotating the sources and the detector arrayrelative to the object.
 25. Radiographic equipment according to claim 1,where the intensity of the first neutron source is of the order 10¹⁰neutrons/second or greater.
 26. Radiographic equipment according toclaim 11 where the scintillators are surrounded by a mask to cover atleast a portion of each of the scintillators, each mask having a firstreflective surface to reflect escaped light pulses back into thescintillator.
 27. The radiographic equipment according to claim 1,wherein the first neutron source has a deuteron energy of less thanabout 200 keV.
 28. The radiographic equipment according to claim 27,wherein the deuteron energy is within a range of about 80 keV to about110 keV.
 29. The radiographic equipment according to claim 1, whereinthe detector array comprises: a first detector array comprising aplurality of scintillator pixels to receive neutron radiation emittedfrom the first neutron source and to convert the received neutronradiation into light pulses; and a second detector array comprising aplurality of scintillator pixels to receive X-ray or gamma-ray radiationfrom the source of X-rays or gamma-rays and to convert the receivedX-ray or gamma-ray radiation into light pulses.
 30. A radiographicequipment comprising: a first source which produces substantiallymono-energetic fast neutrons by a deuterium-tritium ordeuterium-deuterium fusion reaction, the first source having a deuteronenergy of less than about 200 keV; a second source which produces X-raysor gamma-rays of a sufficient energy to substantially penetrate anobject to be imaged, the second source being physically separated fromthe first source; a collimating block which surrounds the first sourceand the second source, the collimating block comprising at least oneslot, each slot for emitting substantially fan-shaped radiation beams; afirst detector array comprising a plurality of scintillator pixels forreceiving neutron radiation which is emitted from the first source andpasses through the object to be imaged, and for converting the receivedneutron radiation into first signals; a second detector array comprisinga plurality of scintillator pixels for receiving x-ray or gamma-rayradiation which is emitted from the second source and passes through theobject to be imaged, and for converting the received x-ray or gamma-rayradiation into second signals; a computing device which determines anattenuation of the mono-energetic fast neutrons and the X-rays orgamma-rays, respectively, based on the first signals and the secondsignals, and generates an output representing a mass distribution andcomposition of the object to be imaged; and a display which displaysimages based on the mass distribution and the composition of the objectto be imaged.
 31. The radiographic equipment according to claim 30,wherein the deuteron energy is within a range of about 80 keV to about110 keV.